Many display apparatuses display images on a display device by using a light unit, which comprises at least one light source for illuminating the pixels of a pixilated display device. Usually, the pixilated display is a matrix display. Usually, in a stable operation state, the light source provides a non-varying light spectrum and the input image is reproduced by modulating the optical state of the pixels. Up till now, predominantly fluorescent lamps are used as the light source. However, LED's, which supply almost monochromatic spectra, are also considered. A known transmissive LCD display comprises pixels made of LC material of which an optical transmission is controlled in accordance with the image to be displayed. In another known reflective DMD display, the pixels comprise small mirrors, which can tilt; an angle of the tilt of the mirrors is controlled in accordance with the image to be displayed. Transflective displays, which partly reflect and partly transmit light from the light sources, are also known.
In a color display device, each one of the pixels comprises sub-pixels and associated color filters to obtain different colors, which together provide the color of the pixel in accordance with the image to be displayed. The colored lights which are leaving the color filters and which illuminate the associated sub-pixels are often referred to as the primary colors of the color display device. These primary colors define the color gamut the display device can display.
For a long time, color display devices used three primary colors, usually red, green and blue. Therefore, almost all input images are defined in a three-component color space, which usually is the RGB color space or a thereto related color space. Recently, the so called multi-primary displays are introduced which use more than three primary colors. It has to be noted that, although “colors” is used in fact is meant different spectrums. Such displays are also referred to as wide gamut displays because a wider color gamut can be displayed by using at least four instead of three primary colors.
Power consumption is an important issue in display apparatus and thus many activities are ongoing to decrease the power consumption. In one of the approaches a wide gamut display, which comprises four sub-pixels per pixel is used in which one of the sub-pixels is white. Usually, the other sub-pixels are red, green and blue, but other colors are possible. It has to be noted that linking a color to a sub-pixel does mean that the light, which is leaving this sub-pixel towards the viewer has the color mentioned.
At a same intensity of the light source, the extra white sub-pixel, which has a transparent color filter, has a much higher luminance than the other sub-pixels because the color filters between the light source and the other sub-pixels suppress a large part of the spectrum. Consequently, the power consumption can be minimized by providing the white part of the color via the white sub-pixel instead of via the other sub-pixels of the pixel. The transparent color filter need not be actually provided but often is present unintentionally because the light leaving the light source has to travel a predetermined distance through the transparent material covering the white sub-pixel.
The use of RGBW display devices with fluorescent lamp as the backlight is limited due to artifacts caused by the RGB to RGBW gamut mapping. In order to make full use of the increased brightness of the RGBW gamut, all the input image components have to be scaled approximately by a factor of two. Unsaturated colors will become two times brighter at the same intensity of the light source, or only half of the intensity of the light source is required to obtain the same brightness. However, saturated colors are scaled outside the RGBW gamut, which leads to undesirable clipping artifacts or unnaturalness after mapping such colors back into the RGBW gamut. These artifacts could be prevented by boosting the intensity of the lamps but this would further increase the power consumption.